The present invention relates to an epoxy resin composition for molding, a molded product thereof and a process for preparing the same, and more particularly to an epoxy resin composition for molding having excellent mechanical properties such as high mechanical strength and high toughness and excellent thermal resistance, a process for preparing a molded product used for apparatus for high voltage made of the epoxy resin composition for molding, and a molded product used for apparatus for high voltage having high crack resistance and high reliability, such as a molded transformer, a molded motor or a gas insulated apparatus, which is produced by the above process.
Conventionally, a molded product used for apparatus for high voltage such as a molded transformer or a molded motor has been produced from an epoxy resin composition comprising a bisphenol A type epoxy resin, an acid anhydride and an inorganic filler such as silica by using a casting method or a pressure gelation method. It is necessary that these molded products do not have voids which cause corona discharge in a molded resin and have characteristics such as high crack resistance for heat cycles and external stress for a long period of time because reliability is required for these molded products for a long period of time.
It is thought that cracks of the molded product are generated by the stress originated in an inserted material embedded in the molded product. The stress .sigma. is briefly represented by the equation: EQU .sigma.=E.times.(.alpha..sub.R -.alpha..sub.I).times.(T.sub.g -T.sub.o)
wherein E is an elastic modulus of a molded resin, .alpha..sub.R is a coefficient of thermal expansion of a molded resin, .alpha..sub.I is a coefficient of thermal expansion of an inserted material, T.sub.g is a glass transition temperature of a molded resin and T.sub.o is the lowest temperature during a heat cycle test. The term "molded resin" is intended to mean a resin used in a molded product.
Conventionally, when an epoxy resin composition is used as a molded resin, mechanical strength of an obtained molded product becomes insufficient. Accordingly, by adding an inorganic filler such as silica to the epoxy resin composition, .alpha..sub.R is lowered, T.sub.g is adjusted to a relatively low temperature as high as about 100.degree. C. and stress is reduced, so that the generation of cracks is restrained and crack resistance is imparted to a molded product. Therefore, if mechanical properties are improved by using the inorganic filler, the thermal resistance of conventional molded products becomes considerably low as high as 70.degree. to 80.degree. C. or so.
Accordingly, in recent years, it has been required that the molded resin is thinned and improved in thermal resistance in the process of miniaturizing of the scale of the molded product and increase of capacity of the molded product. Therefore, it has been eagerly desired to develop a resin composition which can provide a molded resin which is excellent in mechanical properties and thermal resistance.
The present invention has been accomplished in consideration of the above prior art.
It is an object of the present invention to provide a resin composition having excellent mechanical properties and high thermal resistance.
It is a further object of the present invention to provide a molded product used for apparatus for high voltage prepared by molding the resin composition, which is excellent in crack resistance, and the like and which has high reliability.
It is a still further object of the present invention to provide a process for preparing the molded product.
These and other objects of the present invention will become apparent from the description hereinafter.